1. Field of the Invention
The present invention relates to a radio wave radar system for detecting a distance or a relative speed between a host vehicle and an object such as a preceding vehicle or an obstacle, and application technologies thereof.
2. Description of the Related Art
Since attenuation of the radio beam is small even in case of bad weather such as rainy or misty weather, and thus the radio wave propagates to a long distance, the radio wave radar has been widely used in the fields of air traffic control, meteorological observation and the like. Recently, in the field of active safety of vehicles, a radio wave radar using millimeter wave bandwidths (hereinafter, referred to as “millimeter wave radar”) for measuring a distance and a relative speed between a host vehicle and a preceding vehicle was studied, developed and commercialized.
There are several modulation methods used in millimeter wave radar, and a radar technology of two-frequency CW method disclosed in Japanese Patent Publication No. 3203600 is typical. The technology (the principle of detection) disclosed in the Patent Publication will be described with reference to FIGS. 14 to 17. FIG. 14 is a block diagram illustrating a constitutional example of a millimeter wave radar of a two-frequency CW method. As shown in FIG. 14, a millimeter wave oscillator 601 radiates a transmission signal 618 from a transmitting antenna 604, the transmission signal 618 being a modulation signal modulated by the modulation circuit 603 which generates a two-frequency CW modulation signal 602 and the modulation signal being modulated to switch two frequencies f1 and f2 alternatively by time division (here, Δf=f2−f1).
The transmission signal having two frequencies is reflected from a preceding vehicle 605, and this reflected signal is input to a receiving antenna 606 as a reception signal 619. At that time, when there is a relative speed between the preceding vehicle 605 and the millimeter wave radar system 600, Doppler frequencies fd1 and fd2 are generated, and the frequencies of the reception signal 619 become f1+fd1 and f2+fd2, respectively. If the reception signal 619 passes through a mixer 608, the reception signal 619 becomes a time-divided signal (intermediate frequency signal (hereinafter, referred to as “IF signal”)) including information of each of fd1 and fd2. The IF signal 614 is amplified by an amplifier 609, and then is divided in directions of two low pass filters LPFa 611 and LPFb 612 by a switch 610 being switched in synchronization with the two-frequency modulation signal 602. The two-frequency CW modulation signal 602 and the switch 610 are controlled by a control unit 616.
A relationship between the two-frequency CW modulation signal 602 shown in FIG. 14 and the IF signal 615 after passing through the low pass filters 611 and 612 is shown in FIG. 15. The IF signal 615 passing through the low pass filters 611 and 612 becomes two kinds of IF signals indicated by trochoidal curve of the time-divided IF signal 614 before passing through the switch 610. These signals are Doppler signals for the modulated frequencies f1, f2. If these Doppler signals are converted into discrete values by an ADC (AD converter) 616, and then the discrete values are analyzed with FFT by a signal processing unit 617, the frequencies fd1, fd2 and the phase differences Φ1, Φ2 can be obtained. The relative speed V between the host vehicle and the preceding vehicle 605 can be obtained by Equation (1).V=C×fd1/(2×f1) or V=C×fd2/(2×f2)  (1)
Herein, C is a propagation speed of a radio wave, and when fd1<<f1, fd2<<f2 and Δf<<f1, fd1≈fd2 can be approximated. Therefore, V≈C×fd1/(2×f0), where f0=(f1+f2)/2.
In addition, a distance R between two vehicles can be expressed by Equation (2).R=C×(Φ1−Φ2)/(4πΔf)  (2)
Herein, as shown in FIG. 16, if the traveling speeds of a host vehicle 801 and a preceding vehicle 802 existing at the fore are V1 and V2 (V1>V2), respectively, the relative speed V is (V1−V2). If the respective Doppler signal frequencies for this relative speed are fd1 and fd2, the frequency spectrum of power obtained by the FFT analysis of the signals is shown in FIG. 17. As shown in FIG. 17, a sharp peak of the power spectrum is observed on the frequency axis corresponding to the Doppler frequencies fd1, fd2. The relative speed V=(V1−V2) and the distance R between two vehicles can be obtained using Equations (1) and (2) on the basis of the frequency information and the phase information at the peak of the power spectrum.
According to the signal processing in the two-frequency CW method described above, a spectrum is detected from the result of the FFT analysis, one spectrum corresponding to one preceding vehicle is observed as shown in FIG. 17, and the relative speed V from the frequency information and the distance R between vehicles from the phase information can be concurrently obtained. Hence, a stable detection of a preceding vehicle can be realized without a complex signal process.